1. Field of the Invention
The present invention pertains to apparatus and a method useful in the preparation of liquid precursor materials, prior to use of the precursor materials in vaporous form for layer deposition on a substrate.
2. Brief Description of the Background Art
Integrated circuit (IC) device fabrication, micro-electromechanical systems (MEMS) fabrication, and biosurface coating applications, all make use of layers of material which are deposited on a substrate for various purposes. In some instances, the layers are deposited on a substrate and then are subsequently removed, such as when the layer is used as a patterned masking material and then is subsequently removed after the pattern is transferred to an underlying layer. In other instances, the layers are deposited to perform a function in a device or system and remain as part of the fabricated device. Chemical vapor deposition is particularly useful in the formation of layers, where activated (e.g. by means of plasma, radiation, or temperature, or a combination thereof) species react either in a vapor phase (with subsequent deposition of the reacted product on the substrate) or react on the substrate surface to produce a reacted product on the substrate.
In applications where the wear on a surface layer is likely to occur due to mechanical contact or fluid flow over the substrate surface on which the layer of coating is present, it is helpful to have the coating chemically bonded directly to the substrate surface via reaction of the species with the surface in order to obtain particular surface properties. The same is typically true with respect to biosurface coatings which may be lifted from a substrate surface due to reactivity with a material contacting the biosurface. Due to the nanometer size scale of some electrical devices, and the use of MEMS and biosurface layers, where the type and properties of a coating layer on a substrate surface provide functionality on a molecular level, a need has grown for improved methods of controlling the formation of a layer on the substrate surface. Historically, these types of layers were deposited from a liquid phase, resulting in limited film property control and loss of device yield due to capillary forces. More recently, vapor-phase deposition has been used as an alternative, to provide improved coating properties.
Application Ser. No. 10/759,857, filed Jan. 17, 2004, and entitled: “Apparatus And Method For Controlled Application Of Reactive Vapors To Produce Thin Films And Coatings” describes the vapor deposition of various layers of coating materials. The present invention is helpful in the preparation of precursors used to form the coatings. The '857 application describes an improved vapor-phase deposition method and apparatus for the application of various layers on substrates. The method and apparatus are useful in the fabrication of electronic devices, micro-electromechanical systems (MEMS), Bio-MEMS devices, microfluidic devices, a biological surface coatings. The layer formation method typically employs a batch-like addition and mixing of all of the reactants to be consumed during formation of the layer. A coating formation process may be complete after one step, or may include a number of individual steps, where different or repetitive reactive processes are carried out in each individual step. Multiple layers may be deposited. The apparatus used to carry out the method provides for the addition of a precise amount of each of the reactants to be consumed in a single reaction step in which a layer is formed. The apparatus may provide for precise addition of quantities of different combinations of reactants during a single step or when there are a number of different individual steps in the coating formation process. The precise addition of each of the reactants is based on a metering system where the amount of reactant added in an individual step is carefully controlled. In particular, the reactant in vapor form is metered into a vapor reservoir (expansion volume) with a predetermined set volume at a specified temperature and pressure to provide a highly accurate amount of reactant. The entire measured amount(s) of each reactant is (are) transferred in batch fashion into the process chamber in which the coating is formed. The order in which each reactant is added to the chamber for a given reaction step is selectable, and may depend on the relative reactivities of the reactants when there are more than one reactant, the need to have one reactant or the catalytic agent contact the substrate surface first, or a balancing of these considerations.
In some instances, it may be necessary to carry out a series of individual vapor delivery steps to provide a complete coating, rather than carrying out one continuous reaction process. For example, all of a precisely measured quantity of one reacting component may be added initially, followed by a series of precisely measured quantities of a second reacting component. In each case all of the measured quantity is added to the reaction chamber. This provides a precise, carefully measured quantity of reactant at a precise time for each reactant.
A computer driven process control system may be used to provide for a series of additions of reactants to the process chamber in which the layer or coating is being formed. This process control system typically also controls other process variables, such as, (for example and not by way of limitation), process time, chamber pressure, temperatures of the process chamber and the substrate to which the coating is applied, as well as temperatures of the vapor delivery lines and vapor reservoirs relative to the temperatures of the precursors.
The apparatus for vapor deposition of layers, including multi-layered coatings, is particularly useful where at least one precursor used for formation of a layer exhibits a low vapor pressure. Typically the precursor is in the form of a liquid. In some instances the precursor may be in the form of a solid which must be heated to form a liquid. At least one precursor, in the form of a liquid or solid, is provided in a supply vessel, typically referred to as a supply cylinder; vapor from the at least one precursor is transferred to at least one precursor vapor reservoir for accumulating a desired nominal amount of vapor and holding the vapor until it is charged to a reaction chamber. A process controller which receives data from a pressure sensor associated with the precursor vapor reservoir compares vapor reservoir pressure data with a desired nominal vapor reservoir pressure, and sends a signal to a device which controls vapor flow from the precursor container into the precursor vapor reservoir, to prevent further vapor flow into the precursor vapor reservoir when the desired nominal pressure is reached. Each precursor vapor used in formation of a layer of coating material is subsequently transferred to a process chamber for vapor deposition of the coating on a substrate present in the process chamber.
To ensure that a coating having desired nominal and uniform physical properties is obtained, it is advantageous to remove impurities from precursor materials which are used to produce the layers. When the layers are produced by controlled deposition from at least one vaporous precursor, it is necessary to remove impurities from the vaporous precursor. U.S. Pat. No. 5,595,603, to Klinedinst et al., issued Jan. 21, 1997, describes an apparatus and method for the controlled delivery of vaporized precursor to a low pressure chemical vapor deposition (LPCVD) reactor. A liquid flow controller is closely coupled with a liquid vaporizer. The liquid flow controller employs a gas-liquid separator down-stream of the flow control element, which is said to assure that an uninterrupted constant velocity flow of liquid enters the high-temperature zone of the vaporizer. The output of the vaporized-precursor delivery system is linked with the gas inlet of an LPCVD reactor. The system is said to provide very precise vapor delivery rates, leading to precisely controlled thin film deposition rates. (Abstract) The disclosure focuses on flow control of precursor liquid to a vaporization source and vaporization of the liquid precursor. There is no discussion regarding the potential presence of undesired impurities in the liquid precursor. There is one embodiment in which the container of liquid precursor is pressurized by a source of inert gas and where it is acknowledged that a quantity of dissolved inert gas is present in the liquid. A gas/liquid phase separator is used for separating the dissolved inert gas from the flow of liquid precursor. The liquid flows from the gas/liquid phase separator to a liquid vaporizer, and from there to the LPCVD reactor.
U.S. Pat. No. 5,772,736 to van Schravendijk et al, issued Jun. 30, 1998, describes a device for removing dissolved gas from a liquid. An apparatus and a method for delivering a liquid are disclosed. A liquid contained in a vessel is subjected to a pressurized gas. Any pressurized gas dissolved in the liquid is removed in a degas module by passing the liquid through a gas permeable tube subjected to a pressure differential. Then the liquid is dispensed by a liquid mass flow controller. (Abstract)
European Patent Application 94302468.7, published as Publication Number 0 622 475 A1 on Nov. 2, 1994, describes a method and apparatus for degassing semiconductor processing liquids. The apparatus comprises a housing and degasser within the housing. The housing also includes fluid inlet/outlet openings connected to the degasser through which the processing liquid is passed. The degasser includes a separator in the form of a circular pipe defining a tortuous (preferably spiral) path for the processing fluid as it passes through the housing. The separator is configured to be pervious to the molecules of the entrapped or dissolved gas but impervious to the molecules of the liquid. Typically the housing can be evacuated to create a pressure differential across the separator.
An article published in Chemical Engineering World, Vol. XXXIV No. 3, March 1999, pp. 167-170, by S. K. Saxena discussed process vessel (vapor-liquid separator) sizing when degassing criteria controls the design of the vessel. This paper particularly relates to vapor-liquid separators where separation in a process vessel is accomplished either by virtue of density difference aided by gravitation force, impingement separation or both.
The design of the vapor-liquid separators described above is relatively complicated, which is due at least in part to the need to have a continuous flow of liquid to a downstream process. The present inventors are co-inventors of the '857 application which was previously discussed above. The present application and the '857 application are both assigned to Applied Microstructures of San Jose, Calif. The precursors used to produce the reactive vapors which form the thin films/layers of the kind described in the '857 application contain impurities. These impurities commonly become entrapped or otherwise present in the reactive vapors produced from the precursors. As a result, when a given amount of reactive vapor is charged to a processing chamber in which the coating is formed on a substrate, due to the impurity content of the vapor, there may not be sufficient reactant to produce full coverage of the substrate surface by the deposited layer. Further, the impurities may affect the chemical composition and physical properties of the deposited layer. Some suppliers package precursors under a nitrogen atmosphere to prevent decomposition of the precursors. This increases the problem of the presence of impurities in the precursors, which impurities become part of the vapor produced from the precursors. There is a need for a reliable and easily managed method of removing impurities from liquid precursor materials used to generate reactive vapors from which a thin film or layer is formed.